Membrane isolation of repeated-use sweat stimulants for mitigating both direct dermal contact and sweat dilution

With the device integration of sweat stimulation, sweat becomes a stronger candidate for non-invasive continuous biochemical sensing. However, sweat stimulants are cholinergenic agents and non-selective to just the sweat glands, and so, direct placement of sweat stimulants poses additional challenges in the possibility for uncontrollable transport of the stimulant into the body and challenges in contamination of the sweat sample. Reported here is membrane isolation of repeated-use sweat stimulants for mitigating direct dermal contact, dilution of the sweat stimulant, and contamination of the sweat sample. The membrane dramatically reduces passive diffusion of the sweat stimulant carbachol by roughly two orders of magnitude, while still allowing repeated sweat stimulation by iontophoretic delivery of the carbachol through the membrane and into the skin. Both in-vivo and in-vitro validation reveal feasibility for reliable integration of sweat stimulants within a wearable device for use periods of 24 h or more. In addition, advanced topics and confounding issues such as stimulant gel design, osmotic pressure, and ionic impurities are speculatively and theoretically discussed.